I agree with Karen. I wouldn't do anything. There is no major medical reason to have steroids injected and to put your child through surgery for a mark is just crazy in my book. Plus, if it's on top of his head, remind your husband that his hair is going to come in and will cover it up anyway. If he seems embarrassed about it, buy a hat for your son to wear. You'll probably want one in the summer anyway to block the sun from his face. You hear so much about how bad steroids are for you, generally speaking. Why would you want to subject your infant son to that? Besides, you state in your bio that you have decided to with-hold on vaccinations for now, why pump steroids in instead?
Good luck. Hopefully some of these points will help convince your husband to wait it out. You can always have it checked every year or six months if you're that nervous about it.